In these years, it has become general practice that a capacitor structure is formed by using a copper clad laminate in a printed wiring board, in particular, in the internal layer portion of a multilayer printed wiring board, in a manner similar to that in circuit shape formation, and the capacitor structure thus formed is utilized as a built-in capacitor. The formation of a capacitor structure in the internal layer portion of a multilayer printed wiring board has permitted omitting the capacitors arranged on the external layer surface, and hence the miniaturization and high densification of the external layer circuits has become possible so that the number of the parts mounted on the surface is reduced and the production of a wiring board provided with fine pitch circuits becomes easier.
A capacitor structure is produced by use of a copper clad laminate as follows: a double-sided copper clad laminate comprising respective copper foil layers on both sides and a dielectric layer interposed therebetween is used, the copper foil layers on both sides are subjected to etching processing to form desired shapes of capacitor electrodes, and the capacitor structure having the dielectric layer sandwiched by the capacitor electrodes on both side is formed in a target position.
The capacitor is demanded to have an electric capacity as large as possible as a fundamental quality thereof. The capacity (C) of a capacitor is derived from the formula C=∈∈0(A/d) (∈0 is the dielectric constant of vacuum). Accordingly, for the purpose of increasing the capacity of a capacitor, any one of the following procedures has only to be adopted: <1> the surface area (A) of the capacitor electrode is made larger, <2> the thickness (d) of the dielectric layer is made thinner, and <3> the specific dielectric constant (∈) of the dielectric layer is made larger.
However, as far as the surface area (A) described above in <1> is concerned, it is hardly possible to make the area of the capacitor electrode larger in a certain constrained area of a printed wiring board because a printed wiring board is demanded to be lighter and more compact in conformity with the recent trend that electronic and electric appliances are being made lighter and more compact. As for the thickness (d) of the dielectric layer to be made thinner as described above in <2>, if the dielectric layer contains a skeletal material such as glass cloth as represented by prepreg, a limit is given to the formation of a thinner layer by the presence of the skeletal material. On the other hand, if the skeletal material is merely omitted while using a conventional dielectric layer constituting material, an inconvenience occurs that a copper layer is destructed in a portion of the dielectric layer where the copper foil is removed by etching, through the shower pressure of the etching solution that is used in etching for forming a capacitor electrode. In view of these circumstances, it has become general to adopt a procedure in which the specific dielectric constant (∈) of the dielectric layer is made larger as described above in <3>.
In other words, the increase of the electric capacity of a capacitor has been intended to be actualized in the following way: in the construction of the dielectric layer, a skeletal material such as glass cloth is taken to be indispensable and accordingly the skeletal material is made to be an unwoven material so that the skeletal material may be made thinner and hence the thickness of the whole dielectric layer may also be made thinner, and a resin, in which a dielectric filler is dispersed to be contained therein, is used as the material constituting the dielectric layer.
However, further larger increase of the capacity of the built-in capacitor comes to be demanded. In order to achieve the large increase of the electric capacity, many studies have been developed on the dielectric fillers used as the constituent materials for dielectric materials; the constituent materials for a built-in capacitor have been demanded in which the thickness of the dielectric layer can be adjusted optionally while the skeletal material contained in the dielectric layer is omitted.
Additionally, as a problem associated with the moisture absorption of the dielectric layer, generation of the migration phenomenon can be quoted. The migration phenomenon in a dielectric layer containing a skeletal material refers to a phenomenon that the copper component of the copper plating layer in the print wiring board and the metallic component constituting the dielectric filler diffuse to migrate electrophoretically along the interface between the skeletal material and the resin layer and thus causes inconvenience of short circuiting between neighboring circuits. It is conceivable that the presence of the skeletal material makes such a phenomenon occur easily. Still more, this is because a resin, in which a dielectric filler is dispersed with a fairly high filling factor, is used in the layer used as the dielectric layer.
In view of the above circumstances, a dielectric filler containing resin for use in formation of the built-in capacitor layer in the print wiring board and a dielectric filler powder to be dispersed therein have been demanded, where the dielectric layer, in a copper clad laminate, for use in formation of the built-in capacitor layer does not contain a skeletal material, can be formed in an optional thickness, and has a strength sufficient to prevent the destruction thereof caused by the shower pressure of the etching solution at the time of etching.